Since the advent of windows in dwellings, man has been attempting to modify heat loss or absorption through these openings and thus conserve energy.
One general category of devices attempting to solve this problem has been the use of radiation barriers applied to window glazing in the form of special heat absorbing glasses or radiation reflective films applied to glass substrates.
The special heat absorbing glasses are relatively inefficient because much of the heat which they absorb is transferred to the interior areas either by conduction and convexion currents or by secondary radiation from the glass. Glass of this type is also relatively expensive to manufacture in that the oxides required to add the heat absorbing qualities to the glass do not melt homogenously and reproducably and thus glazing of a constant quality is not practically obtainable.
Solar radiation controlled by the application of reflective films to glass substrates has met with reasonable success in preventing solar energy from entering through windows but their use prevents solar energy from warming dwellings during cool seasons. For instance, Gelber in U.S. Pat. No. 3,990,784 on "Coated Architectural Glass System And Method" issued Nov. 9, 1976 teaches the concept of providing a plurality of thin films on a glass substrate which allow a reasonable amount of light transmittance while reflecting solar energy to shield a dwelling from its heating effect. Hunter in U.S. Pat. No. 3,977,930 on "Oil Based Adhesion Of Film To Window" issued Aug. 31, 1976; Franz in U.S. Pat. No. 3,935,351 on "Multiple Glazed Windows Including Selective Reflecting Metal/Metal Oxide Coatings" issued Jan. 21, 1976; and Mochel in U.S. Pat. No. 3,202,054 on "Radiation Filter With Plural Iridized Metal Oxide Films" issued Aug. 24, 1965 are further examples of the application of solar reflective films to glass substrates adapted to prevent solar radiation from entering dwellings. These devices are relatively efficient for their proposed use, that is preventing solar radiation from entering a dwelling and therefore they result in significant energy savings when utilized during the warmer seasons in structures that are air-conditioned. However, their use results in significant energy waste during the cooler seasons because they prevent solar radiation from heating the associated dwelling.